Don't GC message pipes that have an in-flight request

sky/examples/file-browser.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <import src="../framework/sky-element/sky-element.sky" as="SkyElement" />
 <import src="../framework/xmlhttprequest.sky" as="XMLHttpRequest" />
 
 <template>
   <style>
@@ skipping 10 matching lines @@
   </template>
 </template>
 <script>
 SkyElement({
   name: 'file-browser',
   url: '',
   files: [],
   directories: [],
   attached: function() {
     this.url = this.ownerDocument.URL;
-    var xhr = this.xhr_ = new XMLHttpRequest();
+    var xhr = new XMLHttpRequest();
     xhr.open('GET', this.url + '?format=json');
     xhr.onload = (function() {
       var listing = JSON.parse(xhr.responseText);
       this.files = listing.files;
       this.directories = listing.directories;
     }).bind(this);
     xhr.send();
   }
 });
 </script>